In recent years, improvements in the fuel consumption of automobiles are being sought from the viewpoint of protecting the global environment, and there are also demands to further ensure collision safety in automobiles. Therefore, strengthening of automobile bodies and reduction in the weight thereof are being promoted. In view of this background, there is a trend toward the application of press-formed products that are made from high strength steel plates having a thin plate thickness to framework components, suspension components or the like that constitute a vehicle body (hereunder, such components are also referred to as “vehicle components”). The strength of steel plates used as the starting materials for the press-formed products is increasing more and more.
The deformability (press formability) of a steel plate decreases as the strength of the steel plate increases. Therefore, it is difficult to obtain a high quality and high strength press-formed product by performing cold press-working. As a measure to overcome this problem, there is a trend toward the adoption of hot stamping (also referred to as “hot pressing” or “press quenching”) as disclosed in, for example, Japanese Patent Application Publication No. 2004-353026 (Patent Literature 1). In hot stamping, a steel plate that is the starting material is heated to, for example, around 950° C., and thereafter is supplied to a press apparatus. The steel plate is subjected to press-working by a press tooling and is simultaneously quenched.
For vehicle components, providing a difference in the plate thickness is effective for achieving a further reduction in weight while ensuring the component performance. The term “providing a difference in the plate thickness” used here refers to changing the plate thickness between a portion that governs component performance and a portion that has little influence on component performance. Conventionally, in order to provide a difference in the plate thickness of a vehicle component, a tailored blank is used as a steel plate that is supplied for press-working. Such a tailored blank is one kind of varying-thickness steel plate, and includes a portion having a large thickness (hereunder, also referred to as “thick-wall portion”) and a portion having a small thickness (hereunder, also referred to as “thin-wall portion”).
Tailored blanks are broadly divided into the categories of tailored welded blanks (hereunder, also referred to as “TWB”) as disclosed, for example, in Japanese Patent Application Publication No. 2005-206061 (Patent Literature 2), and tailored rolled blanks (hereunder, also referred to as “TRB”) as disclosed, for example, in Japanese Patent Application Publication No. 2002-316229 (Patent Literature 3). A TWB is obtained by joining together a plurality of steel plates having different plate thicknesses and the like by welding. On the other hand, a TRB is obtained by varying the plate thickness by adjusting a gap between rolling rolls that form a pair when producing a steel plate.
However, in a TWB and a TRB, a plate thickness difference between a thick-wall portion and a thin-wall portion is not particularly large. In other words, a ratio “t1/t2” between a plate thickness t1 of the thick-wall portion and a plate thickness t2 of the thin-wall portion is, at most, merely around 1.8. In addition, it cannot be denied that local strength variations that are attributable to welding occur in a TWB. In a TRB, the sizes of the respective regions of a thick-wall portion and a thin-wall portion must be reasonably large. Consequently, the degree of design freedom with respect to vehicle components is low. Accordingly, there is a limit to the degree to which the weight of a press-formed product can be lightened using a tailored blank.